The appearance of a colored gem is a combination of many separate factors, each of which is related to, and affected by, the others. It is precisely the complexity of these intertwined relationships that has bedeviled all attempts to quantify quality. And yet, every time a dealer buys a gem, a quick mental analysis is made, usually within seconds. In grading any gem, one must be cognizant of, but not become lost in, the details.

In the following essay, we will examine in detail the features that make up a fine colored gemstone.

The four C’s

Determining the quality of a gemstone involves what jewelers refer to as the “four C’s.” They are as follows:

Color

Clarity

Cut

Carat Weight

To these, we can also add a fifth C, Color While these factors are well defined for diamond, no universally-accepted system exists for colored gems.

Color: The First C

For a colored stone (any gem other than diamond), color is the most important factor in determining quality.

To the color scientist, given an opaque, matt-finished object, there are three dimensions to color:

Hue position

Saturation (intensity)

Tone (lightness or darkness)

For colored gemstones, there is also a fourth factor:

Color coverage

Hue position.The
position of a color on a color wheel, i.e., red, orange, yellow,
green, blue

The relationship between tone (lightness)
and saturation.

and violet. Purple is intermediate between red and
violet. White and black are totally lacking in hue, and thus achromatic
(‘without color’). Brown is not a hue in itself, but
covers a range of hues of low saturation (and often high darkness).
Classic browns fall in the yellow to orange hues.

Generally
speaking, gems with hues that
most closely resemble the red,
green and blue (RGB) sensors
in our eyes are most popular.
Thus the colored gem trinity,
ruby, emerald and sapphire.
But there is much about hue
that is a personal preference
and will depend upon an individual’s
personal taste.

Three green gems, showing a variation in hue
position. The round center stone is a straight green, while the
trillion-cut stone at left is a more yellowish green and the
oval stone at right a slightly bluish green. Generally speaking,
hue position is of less importance than saturation. Photo: Wimon
Manorotkul

Saturation (intensity). The
richness of a color, or the degree to which a color varies from
achromaticity (white and black are the two achromatic colors, each
totally lacking in hue). When dealing with gems of the
same basic hue position (i.e., rubies, which are all basically
red in hue), differences in color quality are mainly related to
differences in saturation, because humans tend to be more attracted
to highly saturate colors. The strong red fluorescence
of most rubies (the exception being those from the Thai/Cambodian
border region) is an added boost to saturation, supercharging it
past other gems that lack the effect.

Four blue sapphires showing a variation in
saturation and tone. Stone 1 possesses a light tone and low saturation.
Stone 2 is close to ideal in both tone and saturation. Stone
3 has greater saturation than Stone 2 in some areas, but its
overall tone is too dark and it shows too much extinction. Stone
4 is so dark in tone that its saturation is reduced. Note that
inclusions are far more visible in stones of light tone than
those of dark tones. Photo: Wimon Manorotkul

Tone. The degree of lightness or darkness of a color, as a function of the amount of light absorbed. White would have 0% darkness and black 100%. At their maximum saturation, some colors are naturally darker than others. For example, a rich violet is darker than even the most highly saturated yellow, while the highest saturations of red and green tend to be of similar darkness. Note that as saturation increases, so too does tone (since more light is being absorbed. However, there reaches a point where increases in tone may result in a decrease in saturation, as a color “blackens.”

When
judging the quality of a colored
gem, tone is an important consideration.
Before buying, it’s always
a good idea to consider the
lighting conditions under which
it will be worn. Look
for stones that look good even
under the low lighting conditions
you find in the evening or
in a restaurant, for these
are typically the conditions
under which fine gems are worn
and viewed. Also view
gems at arm’s length
and look for those that are
attractive even at a distance.
Exceptional gems will look
great under all lighting conditions
and viewing distances.

Clarity: The Second C

Clarity is judged by reference
to inclusions. Magnification can be used to locate inclusions,
but with the exception of inclusions which might impact durability,
only those visible to the naked eye should influence the final grade. In
this way, colored gems are very different from diamond. Indeed, in
certain cases (Kashmir sapphires being a classic example), the inclusions
can actually enhance beauty and value.

Different levels of clarity are visible here
in these spessartine garnets from Nigeria. The oval stone at
left is eye clean, i.e., with no clarity defects visible to the
unaided eye. In the pear-shaped middle stone, obvious clarity
defects are visible, while in the trillion-shaped stone at right,
they are even more obvious. Photos: Wimon Manorotkul

There
are two key factors in judging clarity. These are:

Visibility
of inclusions

Size: Smaller inclusions are less distracting,
and thus, better.

Number: Generally, the fewer the inclusions,
the better.

Contrast: Inclusions of low contrast (compared
with the gem’s RI and color) are less visible, and thus, better.

Location: Inclusions
in inconspicuous locations (i.e., near the girdle rather than
directly under the table facet) affect value less. Similarly,
a feather perpendicular to the table is less likely to be seen

Impact
on durability

Type: Unhealed cracks may not only be unsightly,
but also lower a gem’s resistance to damage. They are thus
less desirable than a well-healed fracture. As already mentioned,
tiny quantities of exsolved silk may actually improve a gem’s
appearance, and thus, value.

Location: A crack near the culet or corner
would obviously increase the chances of breakage more than one
well into the gem. Similarly, an open fracture on the crown is
more likely to chip than one on the pavilion. Inclusions in certain
positions may also reflect, making a single inclusion visible throughout
a gem.

Note
that cabochon-cut gems generally have poorer clarity than faceted
gem. This is because inclusions are more visible in faceted stones
than in cabs.